This kind of apparatus comprises in general an electric motor, the shaft of which is oriented downwards and which drives a propeller capable of propelling the water, a lower guide pipe or cone, a stabilizer, an upper deflector and at least one float designed to keep the apparatus on the surface of the water.
Depending on the direction of rotation of the propeller, the apparatus can be used either as a mixer by sucking the water which is situated at the surface of the tank towards the propeller which propels this water towards the bottom of the tank, or as an aerator by sucking the water from the bottom of the tank and propelling it onto the surface of the water.
It will be understood immediately that, in order to perform these two different operations, it is necessary to be able to adjust the immersion level of the apparatus relative to the surface of the water in order to obtain high degrees of efficiency.
Floating mixers or aerators designed for the treatment of waste water are known in a general way. Thus, for example, Belgian Patent BE 884,216 describes a device for mixing gas with a liquid, or vice versa, and also for degassing a liquid, consisting of a power generator driving a shaft equipped with a propeller which is situated below the surface of the water, while a blade wheel is fixed to the part of the shaft situated above the propeller.
A similar apparatus has been described in Patent BE 893,687 and is characterized by the fact that the propeller and the blade wheel form part of a single combined rotor body, the lower part of which consists of an axial-propeller pump and the upper part of which consists of a centrifugal rotor.
For the treatment of waste water, it is however, advantageous to saturate the water periodically and alternately with oxygen or to mix the former simply without special addition of oxygen.
In this way, it is possible for the processes, which are known to any specialist and consist in the alternate nitrification and denitrification processes, to be controlled more easily while keeping them under supervision. The consequence of this is that the total quantity of oxygen which is necessary for the treatment of the waste water can be substantially reduced.
Document DE-A-2,119,638 discloses a surface aerator fitted with a float comprising two independent chambers, one of which is connected to a liquid or air supply pipe so as to be able to fill or empty one chamber of the float, thus allowing a height adjustment of the assembly. This height adjustment relative to the water level is, however, exclusively linked to the adjustment of the water throughput and of the oxygen supply required.
Patent AT-PS-378,167 describes a floating surface aerator which can be transformed into a mixer by controlled immersion of the assembly. In order to do this, it is fitted with a device for adjusting the immersion level which employs a float mounted in the central position which can be filled with ballast as desired and which is stabilized by at least one additional float a considerable distance away and connected in a flexible manner to cross-members, for example by means of a chain. The result is then that, if the immersion is sufficient, the float or floats situated apart have a stabilizing effect on the mixer/aerator via the cross-member and the taut chain.
An apparatus of this kind has, however, serious disadvantages.
Namely, the separated additional floats always float on the surface of the water and therefore obstruct the ejection path of the water which is distributed over the surface of the water during the aeration phase. The result is a large decrease in the efficiency and also the stability owing to the appearance of uncontrolled eddies around the floats. Moreover, the addition of cross-members, chains and additional separated floats makes the apparatus more costly, mechanically more sensitive and more difficult to maintain.
If the mixer-aerator is partially submerged by filling the central float with ballast, the device is then stabilized only from the moment when the separated floats are able to exert a force on the cross-members, that is to say from the moment when the chain for joining the cross-members to the float is taut.
When the mixer-aerator is in the floating position, their stabilizing function does not exist and this situation continues as the immersion progresses until the joining chain is taut. Any displacement of the ballast in the fillable float, for example because the liquid filling the fillable float is displaced by a wave or by the effect of the wind, causes a large destabilization of the device, which has to be compensated, in a preferred embodiment of the invention, by placing perforated intermediate partitions in the fillable ballast tank.
In order to prevent the air from being sucked directly through the propeller when the device is immersed and operating as a mixer, it is necessary to arrange additionally a protective plate between the surface of the water and the propeller. This protective plate, mounted above the fillable float, makes the stabilization of the device even more problematical in view of the fact that, whatever the degree of immersion, it displaces the center of gravity of the device even further above the center of flotation of the central float; in other words, the head of the device is thereby made heavier. It is above all during the intermediate phases of the immersion where the floats fixed to the cross-members are still not able to exert any stabilizing action that an arrangement of this kind is a disadvantage, especially as uncontrolled movements of the ballast in the fillable float may further aggravate the situation. Moreover, the protective plate, which is brought into a position where it is to a certain depth completely below the surface of the water in the event of filling, constitutes a serious obstacle with respect to returning the device to the floating position, in view of the fact that all the water which is situated above this plate has to be driven out laterally before the unit can emerge from the water again in order to operate as an aerator.